An encounter doesn’t have to include a threat. A social encounter is an encounter that can be avoided, even though the only threat might be being bored because of dull conversation.
I didn't say an encounter had to include a threat. I said the example you provided... tracks of a potentially hostile group... consisted only of a potential threat and nothing more. It's not really what I think people would consider an encounter. It might lead to one... perhaps the GM has an ambush mapped out with the hostile creatures ready for the PCs only a mile away.
But since you mentioned this example in connection with how it would work in a PbtA type game... that this was a soft move to announce future badness... I don't think of it as an encounter. What's the encounter? It's a hint of a possible threat.
What is there to understand? There is no conflict. As a GM I have to do some prep unless I do 100% improvise. That doesn't mean that I have preplanned goals, outcomes, I don't care what the characters say or do. All it means that if the characters walk into a town I know what's there for them to interact with.
I find there to be conflict. It's not like I'm some noob who doesn't get it. I've been GMing for decades.
So, something that really highlighted this for me was about 5 years ago when the pandemic hit. My group had been playing 5e regularly at that point. I try to run even 5e pretty loosely. I have a big Chessex battle map, and so when the PCs run into trouble of some sort, I draw out a map of the area and we drop minis on it, and we play the combat out.
When the pandemic hit and my group started playing remotely, the requirement of VTT integration was something that really highlighted things for me. I needed to create encounters for the VTT. I needed maps... maps of specific places. I needed stat blocks for the enemies. I needed to load all these into the encounters on the VTT before play would begin. I found that this really clashed with the way I liked to play... where things were loose and not so defined ahead of time.
Now, I realize that I'm saying there is a range of ways to play... and a range to which prep may conflict with player agency, but the extreme circumstances of the pandemic highlighted it for me, and then I started examining the idea in more depth.
Now, you may dismiss this out of hand as not a concern for you, and that's fine. Maybe such a conflict doesn't matter to you. But to say it does not exist... that there is objectively no such conflict? That goes against my experiences.
Because there isn’t. Or rather, there isn’t always.
I prep NPCs with their own interests and goals. The players may choose to interact with those NPCs, or they may not. The fact that there’s an NPC that exists doesn’t force the players to do anything. My Level Up party has no healers. I knew the players were going to a location so I created an NPC cleric and her obvious shrine. The party took damage en route to this location, heard me describe the shrine when I described the location, and didn’t go inside.
Yes, a GM can prep a game and railroad the group. But the GM can also completely improvise and still railroad the group simply by only producing possibilities they want. Heck, I’ve read posts on r/rpghorrorstories that, if true, have players railroading their group by bullying the other players or taking actions that keep the other players from doing what they want. Many people are too conflict-avoidant to say no to this behavior.
We don't need to go to the extremes of railroading to find conflict. There's a whole middle ground where these things can conflict in less severe ways, which may or may not matter to any given group.